And just where does the person get the information in the first place.
I think what people fail to see is the difference between being familiar with a written work to recognize errors in implementation of it, and correcting a written work based on oral teaching.
Since the written work is WRITTEN down, it can be referred back to for veracity. That cannot happen with oral tradition unless that oral tradition is immediately and faithfully recorded. At which point, it becomes a written work that can be checked for fidelity.
oooops.
Even the words of Jesus as written down.........